Heya Nick,
Personally I feel that getting your social worker on your side would be a great place to start, explain to them what you are going through and make it clear that you are concerned about how your brother will cope with the change but more importantly get the point across that you still want to see him and have contact. Just take things slow and try and work with the social worker to get what you want.
I really hope it wouldn't come to this but there may well be a chance that he does struggle with the change, but its worth keeping in mind that people can react that way even when they are not in care. Have you talked about things with anyone else in your family, is there anyone who can give you support or advice there ?
Becky
xx